If you have a string like Hello, my name is John and I'm 25 years old., how can you extract the name and age into separate variables from this string?

There are various methods to achieve this. A simpler method is to use some form of a text matching. However, we can enhance this text matching idea by using a template string such as Hello, my name is {name} and I'm {age} old.. This allows us to capture data dynamically and assign these values to particular variable names, like { "name": "John", "age": "25 years"}. This approach improves portability.

Let’s identify and break down the problem into smaller tasks.

1. Analyze the template string and extract variable names with their locations.
2. Use available location information to extract values from the input string.

1.a. Extract variable names from template string

final regex = r'\{(\w+)\}';
final template = "Hello, my name is {name} and I'm {age} old.";

List<String?> vars = RegExp(regex)
    .allMatches(template)
    .map((e) => e.group(1))
    .toList();

print(vars); // ["name", "age"]

By using a Regex with a capture group, \{{2}(\w+)\}{2}, we can extract matching values. Accessing group(1) of each match allows us to get the result without the surrounding curly braces.

1.b. Identify variable locations

This can be achieved by splitting the text from occurrences of the variables.

List<String> parts = template.split(RegExp(regex));

print(parts); // ["Hello, my name is ", " and I'm ", " old."]

Using the same Regex, we can split the string into multiple sections.

2. Creating the parser

To extract values from the input string, iterate through the previously extracted variables list. Use the parts list to find the start and end positions of the value within the input string that corresponds to the current variable. Add the extracted value to a map, using the variable as the key. This creates a map of extracted key-value pairs.

final result = <String, String>{};

String remainder = input;
int index;
for (var i = 0; i < vars.length; i++) {
  remainder = remainder.substring(parts[i].length);
  index = remainder.indexOf(parts[i + 1]);
  result[vars[i]] = remainder.substring(0, index);
  remainder = remainder.substring(index);
}

if (result[vars.last]!.isEmpty) result[vars.last] = remainder;

print(result); // {"name":"John", "age":"25 years"}